Android application as a service without activity

Sure! No reason you cannot have an application with only a service. …and no need to get into AIDL unless you want to.

The problem is, how to make the application run. When you create an application with an Activity, you add an Intent filter, in the manifest, that makes the activity startable from the Launcher. If there’s no activity, you’ll have to find another way to start it.

It is easy to do, though. Just fire an intent from one of your other programs, like this:

startService(new Intent("my.service.intent"));

… where the service is registered your manifest, like this:

        <service android:name=".SomeService" >
          <intent-filter>
            <action android:name="my.service.intent"/>
          </intent-filter>

You could use that intent to pass Parcelable parameters to the service, and the service can reply by broadcasting intents back.

Of course startService and broadcastIntent are a bit clunky if you really need a complex API between applications and your service. If you need something richer, you will want to look into AIDL and a Bound Service.

Edited to add Intent Filter